1. Field of the Invention
The invention relates generally to an apparatus, and a corresponding method, for chemically etching copper foil-clad substrates, such as copper foil-clad printed circuit board substrates.
2. Description of the Related Art
Thin conductive metal foils, such as thin copper foils, play an important role in the fabrication of printed wiring circuits on printed circuit boards. That is, such printed wiring circuits are typically formed by attaching, e.g., laminating, a thin copper foil to a printed circuit board substrate. The thin copper foil is subsequently, for example, selectively etched to form the desired circuit pattern. This is accomplished by initially forming a patterned photoresist on the copper foil and then etching the copper foil while using the patterned photoresist as an etch mask. Alternatively, a circuit pattern is formed by selectively depositing additional copper onto the copper foil. In this alternative, a patterned photoresist is also initially formed on the copper foil. Then, additional copper is selectively deposited, e.g., electroplated, onto the copper foil, portions of which copper foil and/or deposited copper are subsequently etched away, and thus sacrificed.
Regardless of whether selective etching or selective deposition is used, it is often important that the above-described copper foil be as thin as possible. For example, if selective etching is used, then the thinner the copper foil, the more closely the etched copper foil will match the patterned photoresist. On the other hand, if selective deposition is used, where the thin copper foil is ultimately etched away, it is important that the etching of the copper foil not damage or degrade the corresponding circuit pattern, which is more readily achievable if the copper foil is truly very thin.
Currently, copper foils having thicknesses as small as 12 micrometers are used in forming printed wiring circuits. However, there is a strong desire in the printed circuit board industry to use even thinner copper foils, e.g., copper foils having thicknesses of 9 micrometers, 5 micrometers or even 3 micrometers.
Various attempts have been made to form copper foils having thicknesses equal to or less than 9 micrometers on printed circuit board substrates. In several such attempts, the copper was sputtered directly onto the substrate. Unfortunately, the heat of the sputtering process was so great as to destroy, deform or undesirably alter the corresponding substrates.
Other attempts to form very thin copper foils have involved chemically treating substrates and then chemically depositing very thin copper foils onto the treated substrates. These attempts proved to be very sensitive to contamination, which led to pin hole voids in the deposited copper. Such pin hole voids are undesirable because they can and do degrade the electrical performance of the corresponding printed circuit board.
Significantly, a new method for forming a very thin copper foil on a substrate is described in U.S. Pat. No. 4,917,758 which issued to K. Ishizuka et al on Apr. 17, 1990. Here, a substrate bearing a relatively thick copper foil is chemically etched with a spray-type etching machine in order to thin the relatively thick copper foil. While the reported variation of the thickness of the copper foil remaining after the etching is within +/xe2x88x922.0 micrometers, the etch rate is less than or equal to 0.3 micrometers per second Such low etch rates require corresponding etch times which are much too long for many commercial applications.
Thus, those engaged in the development of printed circuit boards have sought, thus far without success, apparatuses and methods for effectively forming very thin copper foils within commercially viable time periods.
The invention involves an apparatus, and a corresponding method, for chemically etching at least a region of a copper foil overlying a substrate, e.g., a printed circuit board substrate, at an etch rate which is greater than 0.3 micrometers per second, or equal to or greater than 0.5 micrometers per second, and even as high as about 3 micrometers per second. (For purposes of the invention, etch rate is defined to be the average thickness of copper etched divided by the total etching time.) Consequently, the corresponding etch times are definitely commercially viable and, indeed, commercially attractive. Moreover, the chemical etching is sufficiently uniform so that at any point, the variation in the thickness of copper etched is within 10 percent, or even within 6 percent, of the average thickness of copper etched over the entire copper foil region. Thus, for example, if the initial average thickness of the copper foil region is 18 micrometers and the initial thickness variability is ignored, and if the thickness of the copper foil region is to be reduced by, for example, 15 micrometers, to produce a desired final average thickness of 3 micrometers, then the corresponding variation in the thickness of the resulting copper foil region will be within 1.5 micrometers (10 percent of 15 micrometers) of the desired final average thickness. Similarly, if the initial average thickness of the copper foil region is 13 micrometers and the initial thickness variability is ignored, and if the thickness of the copper foil region is to be reduced by, for example, 10 micrometers, to produce a desired final average thickness of 3 micrometers, then the corresponding variation in the thickness of the resulting copper foil region will be within 1 micrometer (10 percent of 10 micrometers) of the desired final average thickness.
Significantly, the inventive apparatus includes a plurality of fluid jet injectors, each of diameter D, which penetrate a surface of the apparatus. Rollers, positioned at the entrance edge and exit edge of the apparatus surface, serve to transport a copper foil-clad substrate over this surface, along a transport direction which is parallel to an imaginary axis associated with the apparatus surface, extending from the entrance edge to the exit edge. As the copper foil-clad substrate moves over the apparatus surface, jets of chemical etchant emitted by the fluid jet injectors impinge upon the substrate, chemically etching the copper foil.
To achieve the above-described etching characteristics, the fluid jet injectors are arranged so that if one were to draw an imaginary line along the apparatus surface in a direction which is parallel to the substrate transport direction, extending from the entrance edge to the exit edge and intersecting the center of at least one fluid jet injector, then the total number of fluid jet injector centers intersected by this imaginary line would be the same as that intersected by any other such imaginary line. In addition, if the distance between any two such adjacent imaginary lines is denoted by S1, then the ratio S1/D should be greater than 0 but less than or equal to about 10, and preferably less than or equal to about 1.3. Moreover, if S2 denotes the distance between any two adjacent fluid jet injectors, then S2/D should be greater than about 2 but less than or equal to about 20, and preferably less than or equal to about 12. It must be emphasized that it is by virtue of this arrangement of fluid jet injectors that the above-described combination of relatively high etch rate and substantially uniform etching is achieved.
It should be noted that the above-described arrangement of fluid jet injectors is based upon the finding that when a jet of chemical etchant impinges upon copper foil, the chemical etchant will etch the copper at an etch rate greater than 0.3 micrometers per second (using the Reynolds numbers, described below) over a circular area having a radial extent of about 10*D. Therefore, in the inventive apparatus, the ratio S2/D is required to be less than or equal to about 20 to ensure that after impingement, any two adjacent fluid jets will overlap each other after spreading out radially no more than about 10*D. In addition, it has been found that when one of the jets of chemical etchant impinges upon a moving copper foil, that the variation in the thickness of the copper etched is within 10 percent of the average thickness of the copper etched only over an area having a width of about 10*D. Therefore, to achieve such a 10 percent uniformity throughout the region of copper foil to be etched, the ratio S1/D is required to be less than or equal to about 10. Moreover, because each of the above-described imaginary lines intersects the same number of fluid jet injector centers, each longitudinal strip of copper foil etched by chemical etchant jets during the passage of the substrate over the apparatus surface necessarily undergoes etching by the same number of chemical etchant jets, over the same total etching time. Consequently, each such longitudinal strip undergoes substantially the same integrated etch rate (integrated over the same total etching time), and therefore substantially the same total amount of etching, resulting in etched copper foil having the relatively low thickness variation, described above.